Why does nitric acid corrode nickel faster than sulfuric acid?

A. Hi, Cia. I am not a chemist but my understanding is that nitric acid is an oxidizing agent at all concentrations whereas sulfuric acid is just a mineral acid, rather than an oxidizing agent, except at very high concentration.

Regards,

A. Stripping nickel in acid is a mass transfer limited process. The nickel nitrate needs to diffuse into the bulk solution and the evolved hydrogen bubbles need to float out of the bath.

If you compare the viscosity of sulfuric acid to nitric acid, you'll notice sulfuric is a lot thicker. Comparing viscosities tends to give you a good idea which liquid has better diffusivity, which lends way to the fact that mass transfer is not happening as readily in sulfuric acid.

A. What Ted says about nitric acid being an oxidising agent is correct. Plus all nitrates are soluble and hence more of the corrosion products go into solution allowing the acid to attack the underlying metal.

A. The redox reaction is nickel metal + nitrate ion react to form nickel cation and nitrite ion. Nickel is oxidized and nitrate (the oxidizing agent in this case) is reduced. This reaction in acidic conditions just happens more favorably compared to a reaction of nickel with sulphate in acidic conditions. You are correct in the sense that it is not a question of solubility or viscosity, it's just a matter of relative rates of reaction in the redox equations.

A. Cia,

Corrosion is not a chemical property. It is the wearing away of a material via dissolution. In a chemical reaction which involves solid/fluid interface, physical properties, such as diffusivity can be very important.

There are two types of corrosion reactions:
-kinetics limited corrosion - this is essentially corrosion which functions under Faraday's law. The anode and cathode will be located separately.

-diffusion limited corrosion - the anode and cathode are located at the same interface. In order for nickel to dissolve further, the nickel nitrate which has formed must first dissolve off of the surface. This is exactly what happens when you dip metal into acid, and why 50% sulfuric etches faster than 98% sulfuric.

Another thing to note is that the oxidation state of nitrogen is +5 for nitric acid and +5 for nickel nitrate. In order for a compound to act as an oxidizing agent, its oxidation state must be reduced as a product of the reaction. NO₃^- is an oxidizer when it becomes NO₂^-, NO, N₂, etc. The oxidizer in both situations is H+, which gets reduced to Hydrogen. The Gibbs free energy of nickel nitrate is lower than that of nickel sulphate, which leads to a greater driving force, but this only gives information about equilibrium and not necessarily kinetics.

From a kinetics standpoint, proton dissociation becomes crucial. In order for redox to occur, a proton must come in contact with the metal. During proton transfer, ion transfer must also occur to minimize electrostatic forces (remember, the reaction site must act as the anode and the cathode all at once). This will happen most readily when the acid has not dissociated. The pka of the first proton on sulfuric acid (sulfuric is diprotic) is about -3. The pka of nitric acid is about 1.6, which means it is around 25 times more likely to occur in its undissociated form than sulfuric acid.